In recent years, the technique of wound repair known as moist healing has become well established. It is an improvement in many cases over the traditional method of letting a wound dry out, forming a scab or crust over the surface, followed by regrowth of tissue underneath the scab. It has been found that, relative to dry healing, moist healing often results in cleaner repair, with less scarring and less pain to the patient than dry healing, especially when the wound is an extensive burn or large abrasion. Dressings for moist healing therapy are frequently made of thin films of synthetic polymers such as polyurethanes as described in U.S. Pat. No. 3,645,835. One of the characteristics of these films is their ability to selectively allow water molecules ("moisture vapor") to pass through them while preventing the passage of liquid water or aqueous solutions and, most importantly, bacteria. By careful selection of film and adhesive, a dressing can be provided which keeps a wound moist and sterile, but which allows excess liquid to evaporate. It also conforms well to the skin, and is unobtrusive in use.
Such dressings, however, have several disadvantages when used with certain kinds of wounds. When a wound is seeping copiously the "moisture vapor transmission" (MVT) capability of the film cannot remove excess liquid fast enough. As a result, fluid may accumulate under the dressing which can result in skin maceration. In practice, a film of sufficiently high MVT to be useful as a dressing on highly exudative wounds would have to be too thin to be practical. Even film dressings in commercial use today are so thin and flimsy that they are extremely difficult to apply without special delivery means such as those described in U.S. Pat. Nos. 4,513,739, 4,598,004 and Canadian Patent No. 1,192,825.
The problem of handling copiously-seeping wounds was addressed in U.S. Pat. No. 4,499,896 by providing a reservoir dressing with one or more extra layers of thin film, sealed together at their peripheries, to form pouches into which excess liquid can flow temporarily. These pouches or reservoirs have additional surface area through which moisture evaporation can take place. These dressings have found utility, but are clearly more complicated and costly than dressings made from a single film.
Another disadvantage of conventional thin film dressings is that they provide very little mechanical cushioning to a wound. Wound protection against bumps and scrapes is not addressed by these thin dressings.
Foam backings for wound dressings are known (e.g. Microfoam.TM. brand surgical tape, 3M Co.) where the foam provides a thicker, more conformable, more cushioning material than would be provided by the same weight of unfoamed backing. The backing of Microfoam.TM. brand surgical tape is open cell polyvinylchloride which is not a barrier for micro organisims. If the polyvinylchloride was made with closed cells, it would not have a sufficiently high MVT for moist wound healing without skin maceration.
U.S. Pat. No. 4,559,938 (Metcalfe) discloses an adhesive dressing comprised of a backing and a conventional pressure-sensitive adhesive. The backing is a film formed from a blend of a continuous matrix of 1,2-polybutadiene and an incompatible polymer which forms a discrete particulate phase within the matrix. This film is stretched to introduce a plurality of small, preferably closed, voids in the film which nominally enhance the moisture vapor permeability of the film. It is believed that the moisture vapor permeability of the dressing (through film and adhesive) is too low to be used in moist wound healing without skin maceration.
Thus, there exists a need for a wound dressing which provides controlled transmission and/or absorption of water vapor away from a wound so that the wound remains moist but not excessively so, and which is also thick and flexible enough to alleviate the need for elaborate delivery means and to provide mechanical cushioning of a wound.